Conventional oil and gas wells have to be drilled in multiple sections in order to ensure that the hydrostatic pressure in the section being drilled lies between the pore and fracture pressures of the surrounding formations. After drilling each section a casing is lowered into place and the annulus between casing and rock is filled with cement. Once set, the cement provides support and protection for the casing and should also provide a seal to prevent fluid communication between the formations through which the well has been drilled. However, cement often does not produce a reliable seal because it is a rigid and brittle material and because standard placement techniques do not provide a mechanism whereby it can be set into a suitable state of stress.
Conventionally, each subsequent section of the well has to be drilled at a smaller diameter in order to allow for its casing to be lowered down through the previous one, so that for very deep wells the reservoir can only be reached with a narrow diameter casing. Recently, expandable casings have been introduced into the market. These are lowered into the well and then expanded in situ to a larger diameter. Using this technology the reduction in diameter with each successive section is reduced, and they can potentially be used to construct a well of constant diameter along its whole depth.
The use of solid expandable tubing in the oil and gas industry is becoming increasingly common. They offer the potential to drill deep wells of significantly larger diameter at the reservoir than can be done with conventional casings.
Expandable tubing pose new challenges for cementing. After expansion, the annulus between the tubing and the borehole is very narrow, which makes it very difficult to achieve satisfactory mud displacement if cement is pumped at this stage. Therefore the cement is normally placed prior to expansion; this means that the expansion must be carried out after placement but before the cement has set, otherwise the rigid set cement will either be crushed by the expansion (potentially fracturing the surrounding rock) or will prevent expansion altogether. The cement must therefore have an unusually long thickening time, as extra time must be allowed to carry out the expansion. It is common practice to cement only the lower portion of each section in this way; if the whole annulus is cemented then an even longer thickening time is required, and this leads to excessively long wait-on-cement times in order to operate in a safe window. However, cementing only the lower section of the casing carries risks also, since it leaves the bulk of the casing unsupported and unprotected against corrosion by formation fluids, and reduces the probability of achieving zonal isolation.
In U.S. Pat. No. 6,431,282, it is proposed to use an elastomeric cladding around the expandable tubing. The elastomeric layer is wrapped around the tubing prior to deployment, and makes a seal with the wellbore wall after expansion. The majority of the radial expansion of the tubing is needed to bring the cladding into contact with the wellbore wall, but if the expansion is large enough the layer may be compressed against the wellbore wall which enhances its sealing ability. In order for this compression to occur, the elastomeric cladding must have sufficient bulk compressibility. Furthermore, since the cladding increases the external diameter of the tubing prior to expansion, it restricts the size of tubing that can be successfully deployed through the previous casing and thus compromises one of the main advantages of the expandable technology. For out of gauge or non-circular wellbores, it is proposed that the cladding should be made from a thermoplastic material which, when heated, will be able to flow sufficiently to fill up all of the annular space. However, this will result at best in the loss of compression of the cladding, and it is not explained how the existing fluid in the wellbore will be displaced by this process.
As an alternative to the external claddings on the casing, the use of in-situ vulcanisable elastomers is suggested in U.S. Pat. No. 6,431,282. Vulcanisable elastomers such as silicones could be pumped into the annulus as fluids and then allowed to set. In one embodiment the expansion occurs prior to the setting of the elastomer, in which case the expansion does not result in the stressing of the material which is necessary to provide a good seal. In another embodiment the tubing is allowed to ‘dip in’ to the unset elastomer, after which the elastomer is set and the tubing expanded. In each case it is not specified whether the elastomer should fill the annulus along the entire length of the tubing or just at its shoe. The filling of the entire annulus with such a material is likely to be prohibitively expensive, but the filling of the annulus only in the region of the shoe leaves the rest of the tubing unsupported and vulnerable to corrosion as described previously.
In the recent application US 2003/0234102, there is proposed the use of a compressible cement-based sealant. This is comprised of cement and an aqueous rubber latex suspension and is foamed by the addition of gas to render it sufficiently compressible. In order to make the material sufficiently deformable to allow for the compression the quantity of latex is more than double that of the cement, which results in a very weak product which is likely to be considerably more permeable than conventional cement both as a result of the foam and the low solid volume fraction of the fluid itself. This material could potentially be used to cement the whole length of the annulus, but given its very large proportion of latex this is also likely to be prohibitively expensive.